1. Field of the Invention
The present invention generally relates to systems and methods for reticle inspection using near-field recovery.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing (CMP), etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on reticles to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices such as ICs. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of semiconductor devices.
Existing methods for reticle inspection utilize one of a number of imaging modes to inspect a mask. The most common inspection mode known as reticle plane inspection (RPI) involves capturing high resolution transmitted and reflected images of a reticle and processing the two images together. Another inspection method called low numerical aperture (NA) inspection (LNI) involves a mode that emulates the wafer scanner optical conditions, capturing one image in transmitted light at a lower NA than RPI with illumination conditions that approximate the scanner's. Another method for contamination inspection known as SL involves analyzing two images, RPI transmitted and reflected, to find defects that standout from the background pattern.
Many reticle inspection methods detect defects on reticles using die-to-database type comparisons. Such inspection typically involves acquiring a microscope image of a reticle. From a database that describes the intended pattern on the reticle, an image that the inspection microscope is expected to observe of that reticle may be calculated or simulated. The acquired optical image may then be compared to the calculated or simulated image to detect defects on the reticle.
Calculating a reticle image as described above may include calculating the diffraction of light by the reticle. Maxwell's equations completely and accurately describe the diffraction of electromagnetic waves by a reticle. However, there is no practical method of solving Maxwell's equations accurately for an entire reticle in the required inspection time, which is one to two hours. Some currently used methods use approximations such as the Kirchhoff approximation to estimate the diffracted field. However, this limits the accuracy of the calculated image and therefore limits the smallest defect that can be detected.
Advancement in optical proximity correction (OPC) results in ever-increasing complexity in the patterns written on a photomask or reticle. Reticle inspection is typically performed using a sophisticated optical microscope such as those described above with advanced algorithms to qualify the written patterns. It becomes more and more difficult to separate critical (printing) defects from non-critical (nuisance) defects due to the OPC complexity. The traditional methods to address this issue are: (a) to utilize empirical rules based on shape and size of the defect for manual disposition, (b) to build empirical rules and automatic defect classification (ADC) software to disposition large numbers of defects automatically, and (c) to use an aerial imaging tool to acquire optical images under the proper lithography conditions to disposition defects.
There are, however, a number of disadvantages of such currently used methods and systems. For example, currently used methods require manual defect disposition by a user to review the defects one-by-one. Increased OPC complexity means oftentimes the user has difficulty separating the printing defects from non-printing defects with a few empirical rules. In addition, potentially large numbers of defects can overload the user and prevent finishing the defect disposition in a reasonable time period. Rule-based methods are not directly related to the printability of a defect. Increased OPC complexity with inverse lithography technology (ILT) can limit the usefulness of the method. In addition, aerial imaging tools utilize a combination of hardware and algorithms to mimic the scanner vector imaging effect. However, the accuracy of such tools is never fully independently verified. More critically, such methods do not consider the complex photoresist development or etching process after imaging is done on a scanner. Studies have shown that there is a poor correlation between the defect size measured on an aerial imaging tool and that measured on a wafer.
Accordingly, it would be advantageous to develop methods and/or systems for reticle inspection that do not have one or more of the disadvantages described above.